On Mon, Sep 21, 2020 at 2:21 AM Daniel Vetter <daniel@xxxxxxxx> wrote: > > On Sat, Sep 19, 2020 at 12:37:23PM -0700, Rob Clark wrote: > > From: Rob Clark <robdclark@xxxxxxxxxxxx> > > > > The android userspace treats the display pipeline as a realtime problem. > > And arguably, if your goal is to not miss frame deadlines (ie. vblank), > > it is. (See https://lwn.net/Articles/809545/ for the best explaination > > that I found.) > > > > But this presents a problem with using workqueues for non-blocking > > atomic commit_work(), because the SCHED_FIFO userspace thread(s) can > > preempt the worker. Which is not really the outcome you want.. once > > the required fences are scheduled, you want to push the atomic commit > > down to hw ASAP. > > > > But the decision of whether commit_work should be RT or not really > > depends on what userspace is doing. For a pure CFS userspace display > > pipeline, commit_work() should remain SCHED_NORMAL. > > > > To handle this, convert non-blocking commit_work() to use per-CRTC > > kthread workers, instead of system_unbound_wq. Per-CRTC workers are > > used to avoid serializing commits when userspace is using a per-CRTC > > update loop. > > > > A client-cap is introduced so that userspace can opt-in to SCHED_FIFO > > priority commit work. > > > > A potential issue is that since 616d91b68cd ("sched: Remove > > sched_setscheduler*() EXPORTs") we have limited RT priority levels, > > meaning that commit_work() ends up running at the same priority level > > as vblank-work. This shouldn't be a big problem *yet*, due to limited > > use of vblank-work at this point. And if it could be arranged that > > vblank-work is scheduled before signaling out-fences and/or sending > > pageflip events, it could probably work ok to use a single priority > > level for both commit-work and vblank-work. > > The part I don't like about this is that it all feels rather hacked > together, and if we add more stuff (or there's some different thing in the > system that also needs rt scheduling) then it doesn't compose. The ideal thing would be that userspace is in control of the priorities.. the setclientcap approach seemed like a reasonable way to give the drm-master a way to opt in. I suppose instead userspace could use sched_setscheduler().. but that would require userspace to be root, and would require some way to find the tid. Is there some way we could arrange for the per-crtc kthread's to be owned by the drm master? That would solve the "must be root" issue. And since the target audience is an atomic userspace, I suppose we could expose the tid as a read-only property on the crtc? BR, -R > So question to rt/worker folks: What's the best way to let userspace set > the scheduling mode and priorities of things the kernel does on its > behalf? Surely we're not the first ones where if userspace runs with some > rt priority it'll starve out the kernel workers that it needs. Hardcoding > something behind a subsystem ioctl (which just means every time userspace > changes what it does, we need a new such flag or mode) can't be the right > thing. > > Peter, Tejun? > > Thanks, Daniel > > > > > Rob Clark (3): > > drm/crtc: Introduce per-crtc kworker > > drm/atomic: Use kthread worker for nonblocking commits > > drm: Add a client-cap to set scheduling mode > > > > drivers/gpu/drm/drm_atomic_helper.c | 13 ++++++---- > > drivers/gpu/drm/drm_auth.c | 4 ++++ > > drivers/gpu/drm/drm_crtc.c | 37 +++++++++++++++++++++++++++++ > > drivers/gpu/drm/drm_ioctl.c | 13 ++++++++++ > > include/drm/drm_atomic.h | 31 ++++++++++++++++++++++++ > > include/drm/drm_crtc.h | 10 ++++++++ > > include/uapi/drm/drm.h | 13 ++++++++++ > > 7 files changed, 117 insertions(+), 4 deletions(-) > > > > -- > > 2.26.2 > > > > _______________________________________________ > > dri-devel mailing list > > dri-devel@xxxxxxxxxxxxxxxxxxxxx > > https://lists.freedesktop.org/mailman/listinfo/dri-devel > > -- > Daniel Vetter > Software Engineer, Intel Corporation > http://blog.ffwll.ch
